The present invention relates broadly to a novel grating structure and to devices incorporating such grating structures.
Optical devices have become increasingly important in the telecommunications field in general. In particular the transmission of data by optical fibers is an attractive alternative to conventional data transmission systems.
Accordingly, there is a great interest in development of optical devices which facilitate e.g. data transfer by optical fibers. Many optical device designs incorporate grating structures for various optical processing functions, including for example for filtering or sensing.
To facilitate the design of new optical devices, it is therefore desirable to provide new grating structures which may allow new functionality in optical devices.
In accordance with a first aspect of the present invention there is provided a grating structure in an optical waveguide, the grating structure being composed of a material having a refractive index variation and the grating structure comprising different order gratings superimposed.
In the context of the present invention the expression xe2x80x9cdifferent orderxe2x80x9d is to be understood as meaning different order with respect to a common wavelength.
A higher order, superimposed grating can result, in use, in the emission of filtered light energy out of the waveguide. This can be utilised e.g. for coupling between waveguides or for introducing a loss mechanism.
In one embodiment, the grating structure may comprise a first and a second order gratings superimposed.
At least one of the different order gratings may be chirped.
At least one of the different order gratings may be sampled.
At least one of the different order gratings may be apodised.
In accordance with another aspect of the present invention, there is provided an optical filter in an optical waveguide, the filter comprising the grating structure of the present invention.
The filter may comprise a chirped second order grating superimposed on a first order grating, the second order grating transmitting, in use, predetermined wavelengths of light energy substantially perpendicular to a core axis of the waveguide and at predetermined positions along the waveguide.
In one application, the filter can be utilized in a spectrographic device. In another application, the waveguide can comprise a distributed feed back laser or distributed Bragg reflectance laser and the filtered light energy forms the emission from the laser. In a further application, the second order grating structure can comprise a series of separate spaced apart second order gratings. In a further application, the grating structure can be formed with a spatially varying amount of zero order modulation along its length.
In accordance with another aspect of the present invention, there is provided an optical free space coupler in an optical waveguide, the coupler comprising a first grating structure in accordance with the present invention.
Preferably, the first grating structure is formed within a first optical waveguide and is arranged to provide the emission of filtered light energy substantially perpendicular to a core axis of the first waveguide; and a second grating structure formed within a second optical waveguide placed in the path of emission of the filtered light energy can couple a portion of the filtered light energy along the second optical waveguide.
At least one of the first or second grating structures may comprise a first order grating and a second order grating superimposed.
The coupler can be used as a sensor when a sample volume is used through which the filtered light energy passes before coupling with the second second order grating. Portions of the first waveguide or the second waveguide are preferably coated with a reflective material which concentrates the filtered light energy along a predetermined path of transmission from the first second order grating to the second second order grating.
In accordance with another aspect of the present invention there is provided an optical sensor in an optical waveguide, the sensor comprising the grating structure of the present invention.
The grating structure preferably comprises a second order grating superimposed on a first order grating formed within an optical waveguide, the grating structure having a predetermined second order modulation so as to provide for the reciprocal emission of optical energy substantially perpendicular to the optical waveguide; the sensor further comprising an optically sensitive material spaced adjacent to the optical waveguide, the material having optical reflective properties variable in accordance with an external physical parameter, the material reflecting the emitted optical energy from the grating structure back to the grating structure.
In accordance with another aspect of the present invention there is provided a device for suppressing ripples in a dispersion compensator in an optical fibre, the device comprising the grating structure of the present invention for providing an optical loss mechanism to effect the suppressing of the ripples.
Preferably, the grating structure comprises a second order grating superimposed on a first order grating.
In accordance with another aspect of the present invention there is provided a dispersion compensator for compensating dispersion in an optical fibre, the compensator comprising the grating structure of the present invention for providing an optical loss mechanism for suppressing ripples.
The grating structure may comprise a second order grating superimposed on a first order grating.
In the aforementioned arrangements, the grating structure can be formed offset from a central axis of the optical waveguide so as to provide directional perpendicular emission. Furthermore, it will be appreciated by a person skilled in the art that other higher order grating structures (i.e. higher than second order) may be utilised. In the description of preferred embodiments given below, calculations are presented for second order gratings (and gratings formed from first order and second order gratings superimposed). It will be appreciated that similar calculations can be performed for higher order gratings, however, it is noted that the loss characteristics will vary between different higher order gratings. E.g. the angular directionality of the loss will differ.